Manufacturing processes which are capable of quickly and cheaply assembling micro-electrical-mechanical systems (MEMS) and other micro-scale devices have not been developed, partly because, at the micro-scale, structures are fragile and easily breakable. They typically break at the micro-Newton (μN) force range- a range that cannot be felt by a human operator assembling microstructure with tweezers and microscopes, and is not reliably measureable by the existing force sensors during assembly. As a result, it is extremely difficult to manipulate parts for assembly at that scale. Moreover, this situation decreases the overall yield and is driving up the cost of MEMS.
Sensing mechanisms have been developed for use in sensing contact force in microassembly/micromanipulation. For example, strain gauges, piezoresistive effect, piezomagnetic effect and capacitive sensors have been developed, with resolutions in the range of sub-mN or mN. Optical techniques have higher resolution, in the range of nanoNewton (nN), but are more expensive and usually have narrow dynamic range. Mostly, PZT-based (i.e., lead zirconium titanate) piezoelectric force sensors have also been developed. Here, more suitably, the resolution of force sensor based on piezoelectric effect is in the range of μN generally.
The present invention addresses the development of a PVDF-based micro force sensing system. With respect to PZT-based piezoelectric force sensors, PVDF has excellent sensitivity and better dynamic properties such as low-Q response, an extremely wide frequency, and broad dynamic range and compliance than the commonly used sensor material PZT. Typically, PVDF has low modulus but with a relatively high electromechanical coupling coefficient. Therefore, the piezoelectric polymer PVDF is an ideal force transduction material for developing a high sensitivity micro force sensing system.
Thus, the present invention affords a current feasible and versatile solution in force sensing for microassembly, through the use of PVDF-based force sensor as a sensing device during the process of microassembly. With respect to the PZT-based piezoelectric force sensors, the piezoelectric polymer PVDF is a more ideal sensing device because of its low-Q response, flexible, light weight, ease of use, easy to shape, and high sensitivity. Based on the piezoelectric effect and the mechanics of material for highly sensitive bending cantilever beam structure, the present invention provides both the models and the structures of the 1-D, 2-D and 3-D PVDF high sensitivity force sensors. By equipping the 1-D, 2-D or 3-D PVDF-based force sensors at the front of the micromanipulator, the micro contact force exerted at the sensor tip can be detected and then extracted and amplified by a processing circuit. Furthermore, the processed signals will be fed back to the controller for regulating the contact/impact force to a safety margin on-line during microassembly.